29/03/2020

Design of Transformer Interview Question Answer - 4


47
Why the clamp less core construction is preferred in the cold rolled oriented steel core?


Clamp less core construction
  • The clamp type core construction distorts the flux path and increases the eddy current therefore the clamp less core construction is preferred in the cold rolled orientation steel core.


48
How the cooling of core is done in the transformer?


Cooling of core
  • The cooling of core is done by providing longitude oil ducts which are running parallel to the lamination or transverse ducts.


49
At which condition, transverse ducts are provided in the transformer core?


Transverse ducts
  • If the capacity of the transformer is very high and circumscribing circle diameter greater than 0.8 meter, the heat flows along the lamination.
  • The transverse ducts improve the cooling of the core in such a condition.


50
Define : Window space factor


Window space factor
  • It is defined as the ratio of copper area to the total window area.


51
Write the output equation of the transformer in terms of volts per turn.


Output equation of transformer
  • Volts per turns = K√Q
  • Where K = √ ( 4.44 × f × r × 103 )
  • Where r = ะคm/AT


52
Describe the condition for minimum cost, minimum losses, minimum weight, maximum efficiency and minimum losses in the transformer.


Minimum cost : Cost of iron is equal to cost of copper
Minimum losses : Variable loss is equal to constant loss
Minimum weight : Weight of iron is equal to weight of copper
Maximum efficiency : Variable loss is equal to constant loss
Minimum losses : Iron loss is equal to copper loss

53
Describe the effect of output and losses in the transformer if the linear dimension of two transformer A and B are in the ratio of X : 1.


The output of the transformer A is X4 times to that of transformer B whereas the losses in the transformer A is X3 times to that of transformer B.

54
Why the value of stacking factor always less than unity?


Stacking factor
  • It is defined as the ratio of net cross section area of core to the gross cross section area of the core.
  • The value of stacking factor always less than unity because the gross area of the core includes some space for insulation coating and oil ducts.


55
Define : Utilization factor


Utilization factor
  • It is defined as the ratio of net cross section area of core to the gross cross section area of the core.


56
What is meaning of the specific iron loss?


It is iron loss per unit volume.

57
Describe the importance of utilization factor.


 Importance of utilization factor
  • The utilization factor increases as the number of step increases.
  • Higher the utilization factor results in increase in core area which will result in increase volt per turn for specific flux density and core diameter.
  • This will result in reduction of winding copper material. Therefore the higher value of utilization factor better for economy consideration.


58
Describe the limitations of number of steps in the transformer.


The maximum number of steps for small rating transformer is limited up to 6 and 15 for large rating transformer.

59
Which type of core construction is preferred in the transformer?


Core type transformer : Rectangular, square or stepped
Shell type transformer : Rectangular

60
Why the circular coils is preferred in the transformer?


Circular coil
  • The circular coil has better mechanical strength.
  • The mechanical stress in the form of circular due to excessive leakage in the case of short circuit.
  • The shape of the circular coil does not change due to radial forces during short circuit whereas the shape of rectangular coil form a circular shape due to forces due to short circuit are perpendicular to the conductors.


61
Why the rectangular core is not used in the large power transformer?


Rectangular core
  • The rectangular coil is not used in the large power transformer because the diameter of circumscribing circle increases which results in wastage of large space. 
  • This will result in length of mean turn of winding increases which produces higher copper losses and cost of conductor.



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